1. Field of the Invention
This invention relates to a data transmission device using a serial bus for the transmission of synchronous data, such as the one defined by the ARINC 659 standard.
It applies notably, though not exclusively, to the transmission of digital data between modules of electronic equipment taken on board aerodynes.
2. Description of the Prior Art
Such a device usually comprises a data transmission bus and couplers which each ensure the connection of an electronic module with the bus and control access to the latter.
The ARINC 659 standard is aimed at specifying an architecture based on a bus, referred to as a "back panel" bus, and bus couplers used in electronic equipment taken on board aerodynes.
According to this standard, the bus comprises four multiplexed and redundant serial data transfer buses enabling transmission errors to be detected and corrected. The couplers have an architecture divided into layers including a layer referred to as "physical" layer, a link layer and a transport layer which ensures the transmission of large volumes of data.
Only the physical and link layers are described by this standard.
Insofar as the physical layer, which groups together the transmission mechanisms and the clock synchronization mechanisms of each coupler, uses the redundancy of the data transmission buses, it has a certain tolerance of faults.
The link layer provides access to the bus and monitors the operations performed by the physical layer. The access principle is essentially based on prior knowledge, on the part of each coupler, of the data exchange traffic on the bus.
To this end, the data is exchanged by means of frames of constant size each comprised of a stream of time windows of variable size during which each coupler takes the initiative of transmitting or receiving a message. This initiative is taken in reference to a table associated with each coupler and defined at the time of designing the device. This table notably mentions the direction of the exchange, the size of the window and the type of data exchanged in this window.
This architecture has the drawback of lacking flexibility since, when one wishes to change the system and in particular the type of data exchanged, the contents of each of the tables associated with the couplers connected to the bus must be revised.
Moreover, at present, a new technique for asynchronous transport of digital data has been implemented to operate telecommunications networks.
This technique, known as ATM (Asynchronous Transfer Mode), is based on the switching of cells of fixed size. According to this transmission mode, the cells are identified by a "virtual channel" number and not by their position in time or rank in a periodical frame, as is the case with synchronous transmission. There is therefore no connection between the data contained in the cells and time.
Each cell thus contains a header containing the virtual channel number and a data field.
These cells are transmitted by time-division multiplexing on the transmission links.
The fundamental advantage of this transmission mode over the synchronous techniques lies in the flexibility of the multiplexing which does not have a rigid, deterministic structure relating to quantified thruputs.